Superconducting driver circuit

ABSTRACT

To obtain a superconducting driver circuit which can obtain an output voltage of several millvolts or above, can use a DC power source as a driving power source, can form no capacitance between it and a ground plane, and has a small occupation area, the superconducting driver circuit is constructed by superconducting flux quantum interference devices (SQUIDs) each constructing a closed loop having as components two superconducting junctions and an inductor. The SQUIDs share the inductors and are connected in series in three or more stages.

BACKGROUND OF THE INVENTION

The present invention relates to the superconducting electronics field.More specifically, the present invention relates to a superconductingdriver circuit of a superconducting flux quantum circuit which uses aflux quantum permitting high-speed signal processing as the carrier of asignal and is used for a measuring circuit for high-speed signalobservation, an analog-to-digital signal converter circuit forhigh-speed analog signal processing, or a high-speed digital dataprocessing circuit.

Two types of prior art driver circuits which are used for the outputpart of a superconducting flux quantum circuit and have a signal voltageamplifying function are considered and are used in a superconductingcircuit. The prior art and embodiments will be described below withreference to the drawings. Only when discriminating between the partsindicated by the reference numerals, numerical subscripts will beidentified as needed.

FIG. 2 is a diagram showing a constructional example of a squid typesuperconducting driver circuit with a control line which has been used.Superconducting flux quantum interference devices each having a closedloop by superconducting junctions 1 and an inductor 7, that is, SQUIDs 6are connected in series and have a control line 21. A signal lineinputting a signal 22 from a superconducting flux quantum circuit isconnected to the control line 21 of the SQUIDs connected in series. Thenumeral 3 denotes a bias current source; the numeral 5 denotes an outputline; and the numeral 8 denotes a ground.

There are two control line wiring methods. In one of the methods, onesuperconducting line is wired to SQUIDs in series as the control line ofall the SQUIDs. In the other method, one signal line is branched to wirecontrol lines to SQUIDs in parallel. When connecting the control linesin parallel, a signal is inputted to the SQUIDs at the same time,thereby enhancing the frequency characteristic.

The flux quantum signal 22 from the superconducting flux quantum circuitis inputted as a current signal to the control line 21. The outputvoltages of the SQUIDs 6 are changed by the current signal. A change inoutput voltage per SQUID is a small value of about 0.1 mV. To increase achange in voltage, ten or more squids are connected in series(“Josephson Output Interfaces for RSFQ Circuits” O. A. Mukhanov et al.,IEEE Transactions on Applied Superconductivity, vol. 7, p. 2826, 1997).

FIG. 3 is a diagram showing another constructional example of asuperconducting driver circuit using superconducting junction lineswhich has been used. One end of a superconducting junction line 100 ₁and one end of a superconducting junction line 100 ₂ connectingsuperconducting junctions 1 in series are connected in parallel viaresistances 31 and an inductor 9. An AC power source 32 supplies an ACvoltage to the midpoint of the inductor 9. An output signal 5 is fetchedfrom the midpoint of the inductor 9. The other end of thesuperconducting junction line 100 ₁ and the other end of thesuperconducting junction line 100 ₂ are grounded. The first stage of thesuperconducting junction line 100 ₁ is connected to an input line toinput a flux quantum signal 22. In the superconducting driver circuit, asuperconducting junction exhibiting the hysteresis characteristic in thecurrent-voltage characteristic is used.

The signal current pulse 22 from a superconducting flux quantum circuitis injected into a superconducting junction 1 ₁ in the first stage ofthe superconducting junction line 100 ₁. When an electric current of thesignal current pulse 22 and a bias current of the AC power source 32exceed a critical current, the superconducting junction 1 ₁ is switchedfrom the superconducting state to the voltage state. The resistancevalue of the superconducting junction in the voltage state is relativelyhigh. The bias current selectively flows to the superconducting junctionline 100 ₂. The current values of the superconducting junctions 1constructing the superconducting junction line 100 ₂ exceed the criticalcurrent. The superconducting junctions 1 constructing thesuperconducting junction line 100 ₂ switched together from thesuperconducting state to the voltage state. The resistance of thesuperconducting junction line 100 ₁ including one superconductingjunction 1 ₁ in the voltage state is lower than that of thesuperconducting junction line 100 ₂ in which the superconductingjunctions 1 switched together to the voltage state. This time, the biascurrent exclusively flows to the superconducting junction line 100 ₁.The remaining superconducting junctions of the superconducting junctionline 100 ₁ all switched from the superconducting state to the voltagestate (“Applications of Synchronized Switching inSeries-Parallel-Connected Josephson Junctions” H. Suzuki et al., IEEETransactions on Electron Devices, vol. 37, p. 2399, 1990).

The superconducting junctions exhibiting the hysteresis characteristicare used in the superconducting driver circuit shown in FIG. 3. Unlessan applied current is lower than a predetermined value, thesuperconducting junctions which once switched to the voltage state arenot returned to the zero-voltage state. When the superconductingjunction exhibiting the hysteresis characteristic switched, a voltagevalue is at millivolt level. It is possible to obtain an output voltagesufficiently higher than 0.1 mV to 0.5 mV as signal voltages of a fluxquantum.

SUMMARY OF THE INVENTION

The characteristic and performance necessary for a superconductingdriver circuit are as follows.

First, an output voltage above several millivolts can be obtained. Inparticular, a semiconductor amplifier exhibits a noise characteristicclose to 1 mV in a high-frequency region above a gigahertz. Asuperconducting circuit chip connected to such a semiconductor amplifiermust have an output characteristic sufficiently higher than the noiselevel of a semiconductor circuit.

Second, as the driving power source of a superconducting driver, a DCpower source can be used. When operating the superconducting driver byan AC power source having the same frequency as an output signal, adriving voltage which is sufficiently higher than a signal voltage of aflux quantum is reversely flowed in superconducting wiring or ispropagated as an electromagnetic wave through the space to be incidentupon a flux quantum circuit, thereby causing malfunction operation. Thepunch-through phenomenon specific for a superconducting junctionincreases the malfunction probability at 10 GHz or above.

Third, a capacitance tends to form between a SQUID and a ground plane.When there is a capacitance component between the boosted portion of thesuperconducting driver and the ground plane, the superconducting driveris charged when being switched from the zero-voltage state to thevoltage state, and is discharged when being switched from the voltagestate to the zero-voltage state. When a boosted voltage is high and thecapacitance component is large, the charge and discharge time is longerto inhibit high-speed operation at a gigahertz or above.

Fourth, the occupation area is small. As the integration scale of asuperconducting flux quantum circuit is larger and the number of outputsignals is higher, it is desired that the occupation area is smaller. Asthe operating frequency is some tens of gigahertz and is higher, thetime in which a signal is propagated in the superconducting driver mustbe 10 picoseconds or below or be sufficiently shorter than this. Forthis, the size of the superconducting driver circuit must be reduced.

To the characteristic and performance required for such superconductingdriver circuit, the prior art superconducting driver circuits used insuperconducting flux quantum circuits have the problems described belowand are hard to pass a necessary sufficient high-frequency signal fromthe superconducting circuit to the semiconductor circuit.

The superconducting driver circuit of the construction shown in FIG. 2has the problems in the first, third and fourth sections. A change inoutput voltage per SQUID is about 0.1 mV. To obtain an output voltage ofseveral millivolts, some tens of squids must be connected in multiplestages. An effort to lower the capacitance is made by the constructionin which only a minimum of ground plane is provided in the SQUID loop.The control line extended from the flux quantum circuit must beconnected to the ground plane. The lowering of the capacitance betweenthe control line 21 and the SQUID inductor 7 is limited. To increase achange in output voltage, as the number of stages of SQUIDs is larger,the size is larger to increase the occupation area.

In the superconducting driver circuit of the construction shown in FIG.3, an alternating current corresponding to the frequency of an outputsignal must be applied as the driving power source 32. Thesuperconducting junction having hysteresis is not returned from thevoltage state to the zero-voltage state unless a source current is oncereturned to a sufficiently small value. The superconducting driver ofthis construction can obtain an output voltage of a millivolt persuperconducting junction. Due to AC driving, the operating frequency ofthe driving power source is an obstacle so that the speed of thesuperconducting driver circuit cannot be increased.

An object of the present invention is to obtain a superconducting drivercircuit having the following characteristic and performance. First, anoutput voltage above several millivolts can be obtained. Second, a DCpower source can be used as a driving power source. Third, capacitanceis hard to form between a SQUID and a ground plane. Fourth, theoccupation area is small.

The present invention takes the following measures for the aboveobjects.

A superconducting driver circuit of the present invention has as a unita superconducting flux quantum interference device, that is, a SQUIDconstructing a closed loop by two superconducting junctions and aninductor in which the SQUIDs share the inductors and are connected inseries in three or more stages. The SQUIDs connected in series areconnected to current bias lines as needed. The current bias lines arealternately connected in the positions near the right and leftsuperconducting junctions of the SQUIDs.

The values of critical currents of the superconducting junctionsincluded in the SQUIDs toward the upper stage (output) side of thesuperconducting junctions constructing the SQUIDs connected in seriesare set to lower. A flux quantum signal is inputted from one or twosuperconducting junction transmission lines to the superconductingjunctions of the SQUID in the lowermost stage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a superconducting driver circuit ofEmbodiment 1 with a superconducting junction transmission line as aninput signal line;

FIG. 2 is a diagram showing a constructional example of a SQUID typesuperconducting driver circuit with a control line which has been used;

FIG. 3 is a diagram showing another constructional example of asuperconducting driver circuit using superconducting junction lineswhich has been used;

FIG. 4 is a diagram showing an example of layout in which thesuperconducting driver circuit of Embodiment 1 is constructed by asuperconducting thin film and an insulator film of oxide;

FIG. 5 is a diagram showing a cross section by noting thesuperconducting junction by the layout shown in FIG. 4;

FIG. 6 is a diagram showing the high-frequency characteristic of outputvoltages of the superconducting driver circuit of Embodiment 1;

FIG. 7 is a diagram showing a superconducting driver circuit ofEmbodiment 2 with a superconducting junction transmission line as aninput signal line;

FIG. 8 is a diagram showing a cross-sectional construction by noting thesuperconducting junction of an example in which the superconductingdriver circuit of Embodiment 2 is constructed by a superconducting thinfilm and an insulator film of oxide;

FIG. 9 is a diagram showing the high-frequency characteristic of outputvoltages of the superconducting driver circuit of Embodiment 2;

FIG. 10 is a block diagram showing a constructional example of asuperconducting circuit in which the superconducting driver circuitaccording to the present invention is coupled to a superconducting fluxquantum circuit;

FIG. 11 is a diagram showing a circuit example of the superconductingflux quantum-voltage converter circuit;

FIG. 12 is a diagram showing a flux quantum waveform in the upper stage,an output voltage waveform of the superconducting flux quantum-voltageconverter circuit in the middle stage, and a voltage waveform of thesuperconducting driver circuit in the lower stage;

FIG. 13 is a block diagram showing another constructional example of asuperconducting circuit in which the superconducting driver circuitaccording to the present invention is coupled to a superconducting fluxquantum circuit;

FIG. 14 is a diagram showing an example of a superconducting fluxquantum multiplier circuit increasing one inputted flux quantum to twofor output;

FIG. 15 is a block diagram showing a further constructional example of asuperconducting circuit in which the superconducting driver circuitaccording to the present invention is coupled to a superconducting fluxquantum circuit;

FIG. 16 is a diagram showing a specific example of a superconductingdriver circuit applied to Embodiment 5 and superconducting junctiontransmission lines for adding a flux quantum signal thereto; and

FIG. 17 is a diagram showing the high-frequency characteristic of outputvoltages of the superconducting driver circuit of Embodiment 5 with fluxquanta inputted.

DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment I

FIG. 1 shows a superconducting driver circuit of Embodiment 1 with asuperconducting junction transmission line as an input signal line. Asuperconducting driver circuit 12 has SQUIDs 6 ₁, 6 ₂, . . . , 6 ₆stacked in six stages. Each of the SQUIDs 6 has superconductingjunctions 1 on the right and left sides and inductors 7 on the upper andlower sides. The SQUID 6 ₁ in the lowermost stage has a loop having aninductor 7 ₁ and two superconducting junctions 1 ₁₁ and 1 ₁₂. The SQUID6 ₂ in the second stage to the SQUID 6 ₆ in the uppermost stage eachhave a loop construction sharing the upper and lower SQUIDs and theinductors 7. For instance, the SQUID 6 ₂ has a loop having two inductors7 ₁ and 7 ₂ and two superconducting junctions 1 ₂₁ and 1 ₂₂. The valuesof critical currents of the superconducting junctions are lower from theSQUID in the lower stage toward the SQUID in the upper stage.Specifically, in FIG. 1, the critical current values of thesuperconducting junctions 1 are 0.25 mA in the SQUID 6 ₁, 0.2 mA in theSQUID 6 ₂, 0.15 mA in the SQUID 6 ₃, 0.1 mA in the SQUID 6 ₄, 0.07 mA inthe SQUID 6 ₅, and 0.05 mA in the SQUID 6 ₆. The critical current valuesare not necessarily fixed to these values or the ratio of these values.

The values of the inductors 7 are larger from the SQUID in the lowerstage toward the SQUID in the upper stage. Specifically, they are 2 pHin the SQUID 6 ₁, 4 pH in the SQUID 6 ₂, 4 pH in the SQUID 6 ₃, 8 pH inthe SQUID 6 ₄, 8 pH in the SQUID 6 ₅, and 16 pH in the SQUID 6 ₆. Thevalues of the inductors are not necessarily fixed to these values or theratio of these values. As a guide of setting, the product of a criticalcurrent by an inductor is a half integral multiplier of a flux quantum(2 fWb). In FIG. 1, the product of the critical current by the inductorof each of the SQUIDs is almost ½ of a flux quantum.

DC bias power sources 3 are alternately connected to the right and leftshoulders of the SQUIDs. That is, the power sources 3 are connected tothe left shoulder of the SQUID 6 ₁, the right shoulder of the SQUID 6 ₂,the left shoulder of the SQUID 6 ₃, the right shoulder of the SQUID 6 ₄,the left shoulder of the SQUID 6 ₅, and the position shifted to theright side from the center of the inductor 7 ₆ in the SQUID 6 ₆ in theuppermost stage. An electric current of each of the SQUIDs applied bythe bias power source 3 has a value corresponding to the differencebetween the critical currents of the superconducting junctions thereofand the SQUID in the upper stage.

To examine the operation of the superconducting driver circuit ofEmbodiment 1, a flux quantum signal as an input signal is introduced viaa superconducting junction transmission line 11 into the left shoulderof the SQUID 6 ₁ in the lowermost stage. The other end of thesuperconducting junction transmission line 11 is provided with a fluxquantum train generation part 4. In the flux quantum train generationpart 4, the superconducting junctions 1 are connected in series to an ACpower source 14 for applying a bias current exceeding the criticalcurrents of the superconducting junctions 1 to produce an AC Josephsoncurrent. The AC Josephson current is propagated as a flux quantum trainin the superconducting junction transmission line 11. The propagationfrequency of flux quanta is increased according to the bias current ofthe AC power source 14. Here, an alternating current in which a finitecurrent value exceeding a zero current and a critical current is anamplitude is used as the AC power source 14 as a bias current source.

FIG. 4 is a diagram showing an example of layout in which thesuperconducting driver circuit of Embodiment 1 is constructed by asuperconducting thin film and an insulator film of oxide. FIG. 5 is adiagram showing a cross section by noting the superconducting junction.In FIG. 4, to easily see correspondence with the circuit of FIG. 1, forsimplification, the insulator film shown in FIG. 5 is omitted.

The superconducting junction 1 of the superconducting driver circuit ofEmbodiment 1 will be described with reference to FIG. 5. Thesuperconducting junction 1 is of a ramp edge type in whichyttrium-barium-copper oxide thin films are used as an upper electrodefilm 45 and a lower electrode film 42. A junction barrier layer 44 is asurface damage layer formed by illuminating the edge surface of thelower electrode film formed with a slope with an ion beam. A singlecrystal and thin film of lanthanum-strontium-aluminum-tantalum oxide areused for a substrate 41 and a interlayer insulator film 43 between theelectrode films. No ground planes are provided.

In the layout of FIG. 4, for instance, the upper electrode film 45 ₁ ofthe SQUID 6 ₁ in the lowermost stage has an inverted U-shaped pattern.The superconducting junctions 1 ₁₁ and 1 ₁₂ are formed between the upperelectrode film 45, and the lower electrode films 43 ₁₁ and 43 ₁₂arranged in both sides of the inverted U-shape. The other end of thelower electrode film 43 ₁₁ and the other end of the lower electrode film43 ₁₂ are connected to wiring 8 as a ground line. The wiring 8 is anecessary pattern as part of the lower electrode film 43. The upper sideof the inverted U-shaped pattern corresponds to the inductor 7 ₁. TheSQUID 6 ₂ in the next stage is formed by the same pattern as the SQUID 6₁. The other end of each of both sides of the inverted U-shape connectedto the wiring 8 as a ground line in the SQUID 6 ₁ is connected to theshoulder portion of the upper side of the inverted U-shaped pattern ofthe SQUID 6 ₁. Here, the portion indicated by the hatching in the lowerleftward direction means that the connecting parts of the upperelectrode film 45 and the lower electrode film 43 are connected bymaintaining the superconducting state. The wiring 3 ₁ to be connected tothe bias power source 3 is drawn out from the left shoulder of the SQUID6 ₁. The wiring 3 ₂ to be connected to the bias power source 3 is drawnout from the right shoulder of the SQUID 6 ₂. The wiring 3 ₆ to beconnected to the bias power source 3 and the output line is drawn outfrom the upper electrode film 45 of the SQUID 6 ₆ in the uppermoststage. The wirings 3 ₁ to 3 ₆ are in a necessary pattern as part of theupper electrode film 45. On the left side of the SQUID 6 ₁ in thelowermost stage, there is the upper electrode film 45 constructing thesuperconducting junction transmission line 11 connected thereto. Thereare shown the superconducting junction constructed by the upperelectrode film 45 and the lower electrode film 43 and the wiring 3 ₁₁connected to the bias power source 3. In comparison of FIG. 4 with FIG.1, it is easily understood that the layout of FIG. 4 is of the sameshape as the circuit.

As described above, for the path 8 of a returning current of thesuperconducting flux quantum circuit, the lower electrode film 43 of thesuperconducting junction are used as wiring. In part of the wiring 3 ₆as an output line 5 of the superconducting driver circuit, in order tosuitably set an impedance, the lower electrode film is used as a groundline to be disposed to be close to the output line 5, which is of acoplanar type. This can obtain impedance matching of the superconductingcircuit chip with the external circuit.

In such a circuit construction, the product of a critical current Ic bya resistance Rn in the voltage state of the superconducting junction,that is, an IcRn value was 2 mV at a temperature of 4.2 K. Thesuperconducting driver circuit exhibited the operation characteristicshown below. When applying an alternating current to the flux quantumtrain generation part, a generated voltage in the superconductingjunction transmission line was 0.8 mV. This corresponds to 400 GHz asthe generation frequency of flux quantum trains. An output voltage ofthe superconducting driver circuit was 4 mV, which was able to amplifythe input voltage to five times.

FIG. 6 is a diagram showing the high-frequency characteristic of outputvoltages of the superconducting driver circuit of Embodiment 1. Analternating current waveform of the AC power source 14 is shown in theupper stage, flux quantum trains generated thereby are shown in the nextstage, and output voltage waveforms of the superconducting drivercircuit are shown in the lower stage. In the drawing, the numeral 51denotes an output voltage of the SQUID 6 ₆ in the uppermost stage, thenumeral 52 denotes an output voltage of the SQUID 6 ₃ in the thirdstage, and the numeral 53 denotes an output voltage of the SQUID 6 ₁ inthe lowermost stage. A higher generated voltage is found to be exhibitedtoward the SQUID in the upper stage. The superconducting driver circuitsufficiently follows the operating frequency of 20 GHz. In the judgmentfrom the steeping of the waveform, this superconducting driver circuitpermits the amplifying function to a signal having a higher frequency.

The operating principle of this superconducting driver circuit is asfollows. In the SQUIDs 6 ₁ to 6₆ connected in series, information on aflux quantum signal is transmitted from the SQUID 6 ₁ in the lowermoststage to the SQUID 6 ₆ in the uppermost stage via the shared inductors7. As a result, the SQUIDs 6 are switched together between thezero-voltage state and the voltage state to generate a voltage obtainedby adding the output voltages of the SQUIDs 6.

The bias current sources 3 supply a DC bias current to the SQUIDs in therespective stages. The SQUIDs are in the zero-voltage state and are heldin the state near the critical point. When a flux quantum passes throughthe SQUID 6 ₁ in the lowermost stage of the superconducting drivercircuit, a loop current with the flux quantum flows into the inductor 7₁. The loop current is added to the bias current. The SQUID 6 ₁ in thelowermost stage is switched to the voltage state.

The electric current added to the inductor 7 ₁ of the SQUID 6 ₁ in thelowermost stage also flows to the loop of the SQUID 6 ₂ in the secondstage. The SQUID 6 ₂ in the second stage is also switched from thesuperconducting state to the voltage state. In the same manner, theSQUID loop current in the lower stage with the inputted flux quantum istransmitted to the SQUID in the upper stage via the inductor sharedbetween two SQUIDs in the upper and lower relation. The SQUIDs connectedin series are sequentially switched to the voltage state. The basicoperation is introduced in the thesis of Kaplunenko et al. (“VoltageDivider Based on Submicron Slits in A High-Tc Superconducting Film andTwo Bicrystal Grain Boundaries” V. K. Kaplunenko et al., Applied PhysicsLetters, vol. 67, p. 282, 1995).

In order that the loop currents of the SQUIDs are sequentiallypropagated and the SQUIDs are switched from the zero-voltage state tothe superconducting state, suitable bias currents must be applied to theSQUIDs. The bias currents all pass through a ground potential or ashared potential to be returning currents. Essentially, the flowingcurrent level is higher toward the SQUID positioned in the lower stage.In response to this, the critical currents of the superconductingjunctions must be higher. By way of example, in Embodiment 1, asdescribed above, the critical current values of the superconductingjunctions 1 are 0.25 mA in the SQUID 6 ₁, 0.2 mA in the SQUID 6 ₂, 0.15mA in the SQUID 6 ₃, 0.1 mA in the SQUID 6 ₄, 0.07 mA in the SQUID 6 ₅,and 0.05 mA in the SQUID 6 ₆.

The superconducting driver circuit according to Embodiment 1 was able togenerate an output voltage of several millivolts in the SQUIDs in sixstages. When using wiring having a width of several microns, thesuperconducting driver circuit can be constructed by the size of 0.1 mmor below. The occupation area of the superconducting driver circuit canbe much smaller than about 1 mm of the prior art SQUID type.

As is apparent from the comparison of the superconducting driver circuitof Embodiment 1 with that of FIG. 2, in Embodiment 1, no magneticcoupling to an input signal is required, that is, the SQUID linesconnected in series are switched by current injection. As compared withthe case of performing magnetic coupling, there is no overlap with amagnetic coupling line grounded. The capacitance component of the groundpotential plane or the ground plane can be small. The superconductingdriver circuits of Embodiment 1 are all driven by a direct current. Thehigh-frequency characteristic of some tens of gigahertz can be easilyexhibited. It can be widely applied to the superconducting flux quantumcircuit.

The superconducting driver circuit of Embodiment 1 can be constructedonly by two superconducting films necessary for constructing thesuperconducting junction and one interlayer insulator film. Thesuperconducting driver circuit can also be constructed by adding onesuperconducting film as a ground plane and one interlayer insulator filmto flow a returning current to the ground plane. In this circuitconstruction, to secure the operation in the high-frequency region, noground planes may be laid other than the region formed with the SQUID 6₁ in the lowermost stage.

In Embodiment 1, for convenience of the layout, the bias currents in therespective stages are alternately supplied from the left and rightshoulders. When setting suitable bias currents, it is apparent that theymay be supplied from the same direction.

In Embodiment 1, the superconducting driver circuit is constructed by asuperconducting thin film of oxide. The superconducting driver circuitcan also be constructed by a superconducting thin film of a metal suchas niobium or niobium nitride.

Embodiment II

FIG. 7 shows a superconducting driver circuit of Embodiment 2 with asuperconducting junction transmission line as an input signal line. Asuperconducting driver circuit 13 has SQUIDs 6 ₂₁, 6 ₂₂, . . . , 6 ₂₄are stacked in four stages. As is understood from the comparison withFIG. 1, the superconducting driver circuit of Embodiment 2 has one SQUID6 ₂₄ in the uppermost stage, two SQUIDs 6 ₂₃ in the next stage, threeSQUIDs 6 ₂₂ in the stage after next, and four SQUIDs 6 ₂₁ in thelowermost stage are connected in parallel, respectively. An inductor 7is shared between the upper and lower SQUIDs. The adjacent SQUIDsconnected in parallel share a superconducting junction.

The critical currents of the superconducting junctions 1 constructingthe SQUIDs are all values almost equal to each other. The values of thesums of the critical currents of the superconducting junctions in therespective stages are lower from the SQUID in the lower stage toward theSQUID in the upper stage. The inductance values of the inductors of therespective SQUID loops are almost equal values. In Embodiment 2, as inEmbodiment 1, the DC bias power sources 3 are alternately connected tothe right and left shoulders of the SQUIDs. That is, the power sources 3are connected to the left shoulder of the SQUID 6 ₂₁, the right shoulderof the SQUID 6 ₂₂, the left shoulder of the SQUID 6 ₂₃, and the positionshifted to the right side from the center of the inductor 7 ₂₄ of theSQUID 6 ₂₄ in the uppermost stage. The applied current of each of theSQUID lines is a value corresponding to the difference between the sumsof the critical currents of the superconducting junctions thereof andthe SQUID line in the upper stage.

To examine the operation of the superconducting driver circuit ofEmbodiment 2, a flux quantum signal as an input signal is introduced viaa superconducting junction transmission line 11 into the left shoulderof the SQUID 6 ₂₁ in the lowermost stage. The other end of thesuperconducting junction transmission line 11 is provided with a fluxquantum train generation part 4. The construction of the flux quantumtrain generation part 4 is the same as that of Embodiment 1.

FIG. 8 is a diagram showing a cross-sectional construction by noting thesuperconducting junction of an example in which the superconductingdriver circuit of Embodiment 2 is constructed by a superconducting thinfilm and an insulator film of oxide. The superconducting driver circuitis the same as that of Embodiment 1 except that a ground plane 46 isprovided via an interlayer insulator film 47. An example of layout inwhich the superconducting driver circuit of Embodiment 2 is constructedby a superconducting thin film and an insulator film of oxide can beconstructed as in the correspondence of the circuit of FIG. 1 with thelayout of FIG. 4 in Embodiment 1. The drawing thereof is omitted.

The ground plane 46 of Embodiment 2 may be provided in the entire regionas a magnetic shield film. In the region portion in which thesuperconducting driver circuit is constructed, the ground plane 46 islaid only in the region portion of the SQUID 6 ₂₁ in the lowermoststage. In the region portion in which the SQUIDs in the second stage tothe uppermost stage of the superconducting driver circuit areconstructed, no ground planes are laid.

In such a circuit construction, the product of a critical current Ic bya resistance Rn in the voltage state of the superconducting junction,that is, an IcRn value was 2 mV at a temperature of 4.2 K. Thesuperconducting driver circuit exhibited the operation characteristicshown below. When applying an alternating current to the flux quantumtrain generation part, a generated voltage in the superconductingjunction transmission line was 0.8 mV. An output voltage of thesuperconducting driver circuit was 2.5 mV or above, which was able toamplify the input voltage to three times or more.

FIG. 9 is a diagram showing the high-frequency characteristic of outputvoltages of the superconducting driver circuit of Embodiment 2. An ACwaveform of an AC power source 14 is shown in the upper stage, fluxquantum trains generated thereby are shown in the next stage, and outputvoltage waveforms of the superconducting driver circuit are shown in thelower stage. In the drawing, the numeral 54 denotes an output voltage ofthe SQUID 6 ₂₄ in the uppermost stage, the numeral 55 denotes an outputvoltage of the SQUID 6 ₂₃ in the third stage, the numeral 56 denotes anoutput voltage of the SQUID 6 ₂₂ in the second stage, and the numeral 57denotes an output voltage of the SQUID 6 ₂₁ in the lowermost stage. Ahigher generated voltage is found to be exhibited toward the SQUID inthe upper stage. The superconducting driver circuit sufficiently followsthe operation frequency of 20 GHz. In judgment from the steeping of thewaveform, this superconducting driver circuit permits the amplifyingfunction to a signal having a higher frequency.

The superconducting driver circuit of Embodiment 2 can be constructed byarraying the superconducting junctions having almost the same size andalmost equal critical currents and inductors having almost the samesize. It is relatively easy to manufacture the circuit as designed. Toincrease the output voltage, when increasing the number of stages of theSQUID line, it is possible to cope with change in design and layoutrelatively easily.

Embodiment III

FIG. 10 is a block diagram showing a constructional example of asuperconducting circuit in which the superconducting driver circuitaccording to the present invention is coupled to a superconducting fluxquantum circuit. The entire superconducting circuit has asuperconducting flux quantum circuit 101, a superconducting fluxquantum-voltage converter circuit 102, and a superconducting drivercircuit 103. These circuits are connected by superconducting junctiontransmission lines 11. The superconducting flux quantum circuit 101 is acircuit performing various kinds of logic processing using a fluxquantum as a signal. The superconducting driver circuit 103 is theabove-described circuit as Embodiment 1 or 2.

FIG. 11 shows a circuit example of the superconducting fluxquantum-voltage converter circuit. The superconducting fluxquantum-voltage converter circuit is a circuit which continuouslygenerates a flux quantum train as an output 5 for each reaching of aflux quantum 22 to be brought to the voltage state and stops fluxquantum train generation by reaching of the next flux quantum 22. Foreach reaching of the flux quantum 22, such flux quantum train generationand stop are repeated. As described above, the superconducting drivercircuit 103 receiving as an input a signal in which such flux quantumtrain generation and stop are repeated amplifies a voltage correspondingto the frequency of flux quantum trains generated by the superconductingflux quantum-voltage converter circuit 102.

In Embodiment 3, the construction of the superconducting driver 103 isalmost the same as that of Embodiment 1. That is, SQUIDs are stacked insix stages, and the SQUIDs have superconducting junctions on the rightand left sides and the inductors are provided in the upper and lowerstages. The inductor is shared between the upper and lower SQUIDs. Thevalues of critical currents of the superconducting junctions are lowerfrom the SQUID in the lower stage toward the SQUID in the upper stage.DC power source lines are connected alternately to the right and leftshoulders of the respective SQUIDs. In the SQUID in the uppermost stage,the power source line is connected to the position shifted from thecenter of the inductor. The superconducting driver circuit isconstructed by a superconducting thin film and insulator film of oxide.Its cross-sectional construction is the same as that shown in FIG. 8.The ground plane and the superconducting junction are of the ramp edgetype in which yttrium-barium copper oxide thin films are upper and lowerelectrodes. A single crystal and thin film oflanthanum-strontium-aluminum-tantalum oxide are used for a substrate andthe interlayer insulator film between the electrodes.

The ground planes are laid in the superconducting flux quantum circuitportion and the path of a returning current thereof, the superconductingjunction transmission line connected to the superconducting drivercircuit, the SQUID in the lowermost stage of the superconducting drivercircuit, and the output line of the superconducting driver circuit. Noground planes are laid in the SQUIDs in the second stage to theuppermost stage of the superconducting driver circuit.

In such circuit construction, the product of a critical current Ic by aresistance Rn in the voltage state of the superconducting junction, thatis, an IcRn value was 2 mV at a temperature of 4.2 K. An output voltageof the superconducting flux quantum-voltage converter circuit was 0.44mV. The output voltage corresponds to 220 GHz as the flux quantumgeneration frequency. A flux quantum train from the superconducting fluxquantum-voltage converter circuit was inputted to the superconductingdriver circuit. An output voltage of 2 mV was obtained by thesuperconducting driver circuit.

FIG. 12 is a diagram showing a flux quantum waveform in the upper stage,output voltage waveforms of the superconducting flux quantum-voltageconverter circuit in the middle stage, and a voltage waveform of thesuperconducting driver circuit in the lower stage. It is found that fluxquantum train generation and stop are repeated for each reaching of theflux quantum 22 to obtain a voltage corresponding to the frequency offlux quantum trains generated by the superconducting fluxquantum-voltage converter circuit 102 from the superconducting drivercircuit 103 receiving as an input a signal in which such flux quantumtrain generation and stop are repeated.

The superconducting driver circuit amplifies an output signal of thesuperconducting flux quantum circuit. It is important that a producedvoltage signal not be reversely flowed to the superconducting fluxquantum circuit. The reverse flow of a voltage pulse becomes noise tothe superconducting flux quantum circuit. The flux quantum traingenerated by the superconducting flux quantum-voltage converter circuit102 of the construction shown in Embodiment 3 is not reversely flowed tothe superconducting flux quantum circuit. The electric current flowed tothe superconducting junction is not particularly varied. This isunderstood from the operation waveform shown in FIG. 12. It is apparentthat the waveform (in the upper stage) of the flux quantum inputted tothe superconducting flux quantum-voltage converter circuit is notaffected at all by the voltage of the superconducting driver circuitshown in the lower stage. In the drawing, no returning current from thesuperconducting driver circuit 103 appears at all. It is found that aproduced voltage (flux quantum signal) in the junction is the same asthe case that the superconducting driver circuit is not connected.

The superconducting driver circuit according to the present invention iscoupled to the superconducting flux quantum circuit via thesuperconducting flux quantum-voltage converter circuit to construct thesuperconducting circuit. It is possible to construct the superconductingcircuit which causes no noise to the superconducting flux quantumcircuit itself and cannot affect the operation margin of thesuperconducting flux quantum circuit.

Embodiment IV

FIG. 13 is a block diagram showing another constructional example of asuperconducting circuit in which the superconducting driver circuitaccording to the present invention is coupled to a superconducting fluxquantum circuit. The entire superconducting circuit has asuperconducting flux quantum circuit 101, a superconducting fluxquantum-voltage converter circuit 102, a superconducting flux quantummultiplier circuit 104, and a superconducting driver circuit 103. Thesecircuits are connected by superconducting junction transmission lines11. As is apparent from comparison of FIG. 10 with FIG. 13, inEmbodiment 4, they are the same except that the superconducting fluxquantum multiplier circuit 104 is inserted between the superconductingflux quantum-voltage converter circuit 102 and the superconductingdriver circuit 103.

As described above, the superconducting driver circuit 103 can obtain avoltage corresponding to the frequency of inputted flux quantum trains.To obtain a high output voltage, it is useful to increase the frequencyof inputted flux quantum trains. The superconducting flux quantummultiplier circuit 104 is inserted for this reason.

FIG. 14 shows an example of a superconducting flux quantum multipliercircuit which increases one inputted flux quantum to two for output. Thesuperconducting flux quantum multiplier circuit has a branching circuit24 to which one flux quantum 22 is inputted, the superconductingjunction transmission lines 11 for propagating two outputs of thebranching circuit 24, and a confluence buffer 25 inputting two fluxquanta outputted from the superconducting junction transmission lines 11to output two flux quantum trains. The confluence buffer 25 has twoinput lines and one output line 5. The flux quanta pass from therespective input lines through the output line. It is possible toprevent the flux quantum from passing from one of the input linesthrough the other input line and the flux quantum from reversely flowingfrom the output line to the input lines. In this circuit, even when thepropagation speeds of the two flux quanta produced by the branchingcircuit 24 are the same, they are outputted as two flux quantum trains.

The superconducting flux quantum multiplier circuit 104 shown in FIG. 14doubles the frequency of flux quantum trains for input to thesuperconducting driver circuit 103. The superconducting flux quantummultiplier circuit 104 is inserted to make the output voltage almostdouble.

As shown in FIG. 14, there is only one superconducting flux quantummultiplier circuit. It is possible to connect two or moresuperconducting flux quantum multiplier circuits in series. When twosuperconducting flux quantum multiplier circuits are arrayed, thefrequency of flux quantum trains is four times. When threesuperconducting flux quantum multiplier circuits are arrayed, thefrequency of flux quantum trains is eight times. When the frequency offlux quantum trains of the superconducting flux quantum multipliercircuit is larger than a value corresponding to IcRn of thesuperconducting junction, the multiplying function is suppressed. Thefrequency of flux quantum trains corresponding to 1 mV of the IcRn valuecorresponds to 500 GHz.

Embodiment V

FIG. 15 is a block diagram showing a further constructional example of asuperconducting circuit in which the superconducting driver circuitaccording to the present invention is coupled to a superconducting fluxquantum circuit. FIG. 16 is a diagram showing a specific example of asuperconducting driver circuit applied to Embodiment 5 andsuperconducting junction transmission lines for adding a flux quantumsignal thereto. The entire superconducting circuit has a superconductingflux quantum circuit 101 and a superconducting driver circuit 103. Thesuperconducting driver circuit 103 is set and reset by a flux quantumsignal outputted from the superconducting flux quantum circuit 101.

With reference to FIG. 16, a specific example of the superconductingdriver circuit applied to Embodiment 5 will be described. The numeral 14denotes a superconducting driver circuit, which has SQUIDs 6 ₃₁, 6 ₃₂, 6₃₃ and 6 ₃₄ stacked in four stages. The values of critical currents ofsuperconducting junctions 1 of the SQUIDs 6 are lower from the SQUID inthe lower stage toward the SQUID in the upper stage. The values ofinductors 7 ₃₁, 7 ₃₂, 7 ₃₃ and 7 ₃₄ are larger from the SQUID in thelower stage to the SQUID in the upper stage. The product of the criticalcurrent by the inductor is almost ½ of a flux quantum.

Except for the SQUID 6 ₃₁ in the lowermost stage, DC power source lines3 are alternately connected to the right and left shoulders of theSQUIDs 6 ₃₂, 6 ₃₃ and 6 ₃₄. In the SQUID in the uppermost stage, thepower source line is connected to the position shifted from the centerof the inductor. The applied current of each of the SQUIDs is a valuecorresponding to the difference between the critical currents of thesuperconducting junctions thereof and the SQUID in the upper stage. Itis different from the superconducting driver circuit shown in FIG. 1 inthat the DC bias power source line 3 is not connected to the SQUID 6 ₃₁in the lowermost stage.

Superconducting junction transmission lines 15 and 16 for input signalare connected to both sides of the SQUID 6 ₃₁ in the lowermost stage ofthe superconducting driver circuit 14. A flux quantum signal 22outputted from the superconducting flux quantum circuit 101 is addedfrom the left end of the superconducting junction transmission line 15.This is inputted to the left side of the SQUID 6 ₃₁ in the lowermoststage of the superconducting driver circuit 14. The superconductingjunction transmission line 16 is branched from the middle portion of thesuperconducting junction transmission line 15 and the flux quantumsignal 22 is introduced thereinto. This is inputted to the right side ofthe SQUID 6 ₃₁ in the lowermost stage of the superconducting drivercircuit 14.

As in Embodiment 1, the superconducting driver circuit 14 of Embodiment5 is constructed by a superconducting thin film and insulator film ofoxide. The superconducting junction is of the ramp edge type in whichyttrium-barium-copper oxide thin films are upper and lower electrodes. Asingle crystal and thin film of lanthanum-strontium-aluminum-tantalumoxide are used for a substrate and an interlayer insulator film betweenthe electrodes.

The superconducting driver circuit 14 of Embodiment 5 is operated by thefollowing procedure. When a flux quantum is inputted from the leftsuperconducting junction transmission line 15, the superconductingdriver circuit 14 is brought to the voltage state. When a flux quantumis inputted from the right superconducting junction transmission line16, the superconducting driver circuit is returned to thesuperconducting state. The flux quantum signals 22 outputted from thesuperconducting flux quantum circuit 101 were inputted from both sidesof the squid 6 ₃₁ in the lowermost stage of the superconducting drivercircuit 14 as flux quantum signals whose frequencies were equal andwhose phases were shifted at a fixed rate by the superconductingjunction transmission lines 15 and 16. In the voltage state, an outputvoltage of 2.5 mV was obtained by an output line 5 of thesuperconducting driver circuit 14.

The operating principle of the superconducting driver circuit 14 ofEmbodiment 5 is as follows. In the SQUIDs 6 ₃₁, 6 ₃₂, 6 ₃₃ and 6 ₃₄connected in series, information on a flux quantum signal is transmittedfrom the lowermost stage to the uppermost stage via the shared inductors7. As a result, the SQUIDs are switched together between thezero-voltage state and the voltage state to produce a voltage obtainedby adding these.

Except for the SQUID 6 ₃₁ in the lowermost stage, DC bias currents areapplied to the SQUIDs 6 ₃₂, 6 ₃₃ and 6 ₃₄ in the respective stages. Therespective SQUIDs are in the zero-voltage state and are held in thestate close to the critical point. When the flux quantum 22 reaches fromthe superconducting junction transmission line 15 to the SQUID 6 ₃₁ inthe lowermost stage of the superconducting driver circuit 14, a loopcurrent with the flux quantum flows to the inductor 7 ₃₁. The biascurrent is not applied to the SQUID 6 ₃₁ in the lowermost stage. TheSQUID 6 ₃₁ is not switched to the voltage state when the loop currentwith the flux quantum only flows. Until the next flux quantum reachesfrom the superconducting junction transmission line 16, the flux quantumof the SQUID 6 ₃₁ in the lowermost stage remains. The loop currentcontinues to flow.

The electric current added to the inductor 7 ₃₁ of the SQUID 6 ₃₁ in thelowermost stage also flows to the SQUID loop of the SQUID 6 ₃₂ in thesecond stage. The bias current is applied to the SQUID 6 ₃₂ in thesecond stage. The loop current is added so that the SQUID 6 ₃₂ isswitched to the voltage state. In the same manner, the loop current withthe flux quantum is transmitted to the SQUID in the upper stage via theinductor shared between two SQUIDs. The SQUIDs connected in series aresequentially switched to the voltage state.

When the next flux quantum reaches from the superconducting junctiontransmission line 16, the flux quantum of the SQUID 6 ₃₁ in thelowermost stage disappears and the superconducting current of the SQUIDloop is zero. Since the loop current does not flow, the SQUID 6 ₃₂ inthe second stage returns from the voltage state to the zero-voltagestate. In the same manner, the SQUIDs connected in series aresequentially switched to the zero-voltage.

In this superconducting driver circuit, one flux quantum is used forswitch operation, and the SQUID in the lowermost stage does not switchto the voltage state. Noise with the switch of the superconductingdriver circuit cannot affect the connected superconducting flux quantumcircuit.

FIG. 17 shows the high-frequency characteristic of output voltages ofthe superconducting driver circuit 14 with flux quanta inputted. Theupper stage shows flux quanta inputted via the superconducting junctiontransmission line 15. This brings the SQUIDs 6 ₃₂, 6 ₃₃ and 6 ₃₄ of thesuperconducting driver circuit 14 to the voltage state. The middle stageshows flux quanta inputted via the superconducting junction transmissionline 16. This returns the SQUIDs 6 ₃₂, 6 ₃₃ and 6 ₃₄ of thesuperconducting driver circuit 14 from the voltage state to thezero-voltage state. The lower stage shows output voltages of therespective SQUIDs of the superconducting driver circuit 14. The numeral58 denotes an output voltage of the SQUID 6 ₃₄ in the upper most stage.The numeral 59 denotes an output voltage of the SQUID 6 ₃₃ in the thirdstage. The numeral 60 denotes an output voltage of the SQUID 6 ₃₂ in thesecond stage. The numeral 61 denotes an output voltage of the SQUID 6 ₃₁in the lowermost stage. A higher generated voltage is found to beexhibited toward the SQUID in the upper stage. An output voltage of 2.5mV was obtained. The voltage generation time was 25 picosecondscorresponding to the reaching time difference of two flux quantaaccording to the delay of the branched superconducting junctiontransmission line 16. The superconducting driver circuit sufficientlyfollowed an operation frequency of 20 GHz. In judgment of the steepingof the waveforms, this superconductor driver circuit permits theamplifying function to a signal having a higher frequency.

This superconducting driver circuit has two superconducting filmsnecessary for constructing the superconducting junction and oneinterlayer insulator film. As in Embodiment 1, the superconductingdriver circuit may be constructed by adding one superconducting thinfilm as a ground plane and one interlayer insulator film to flow areturning current to the ground plane. In this circuit construction, tosecure the operation in the high-frequency region, no ground planes arelaid other than the SQUID in the lowermost stage.

The superconducting driver circuit having the following effects can berealized.

(1) The superconducting driver circuit can be reduced in circuit areaand size and can be used in a highly integrated circuit. Thesuperconducting driver circuits can be arrayed corresponding to manyoutput signals.

(2) The superconducting driver circuit uses a DC power source as a powersource. The timings of a flux quantum signal and the driving powersource need not be matched with each other so that the circuit operationis easy. Without limiting the operation frequency, the superconductingdriver circuit can cope with a high-frequency region of some tens ofgigahertz.

(3) The boosted SQUID portion need not be overlapped with the groundplane. Without lowering the processable operation frequency, thesuperconducting driver circuit can cope with a high-frequency region ofsome tens of gigahertz.

(4) No noise from the superconducting driver circuit to thesuperconducting flux quantum circuit exists. The operation margin of thesuperconducting flux quantum circuit cannot be reduced.

1. A superconducting driver circuit for voltage amplificationcomprising: a superconducting flux quantum interference device in afirst stage constructing a closed loop having as components twosuperconducting junctions and an inductor; a superconducting fluxquantum interference device in a second stage constructing a closed loophaving as components two superconducting junctions and an inductor bysharing said inductor; and a superconducting flux quantum interferencedevice in a third stage constructing a closed loop having as componentstwo superconducting junctions and an inductor by sharing the inductor ofsaid superconducting flux quantum interference device in said secondstage, wherein the superconducting junctions of the superconducting fluxquantum interference devices toward a lower stage of the superconductingjunctions of the superconducting flux quantum interference devices insaid respective stages have a larger critical current value, apredetermined bias current is supplied to the superconducting junctionsof the superconducting flux quantum interference devices in saidrespective stages, and a flux quantum signal is inputted to thesuperconducting flux quantum interference device in the first stage toobtain an output voltage from the superconducting flux quantuminterference device in the third stage.
 2. The superconducting drivercircuit for voltage amplification according to claim 1, wherein thesuperconducting flux quantum interference devices are provided in fouror more stages, and an output voltage is obtained from a superconductingflux quantum interference device in the uppermost stage.
 3. Asuperconducting driver circuit for voltage amplification comprising:superconducting flux quantum interference devices in a first stage inwhich a plurality of superconducting flux quantum interference deviceseach constructing a closed loop having as components two superconductingjunctions and an inductor are arrayed in parallel so as to share anadjacent superconducting junction; superconducting flux quantuminterference devices in a second stage in which a plurality ofsuperconducting flux quantum interference devices each constructing aclosed loop having as components two superconducting junctions and aninductor are arrayed in parallel in number one fewer than thesuperconducting flux quantum interference devices in the first stage soas to share said inductor and to share the adjacent superconductingjunction; and superconducting flux quantum interference devices in athird stage in which a plurality of superconducting flux quantuminterference devices each constructing a closed loop having ascomponents two superconducting junctions and an inductor are arrayed inparallel in number one fewer than the superconducting flux quantuminterference devices in the second stage so as to share the inductor ofsaid superconducting flux quantum interference device in the secondstage and to share the adjacent superconducting junction, wherein thesuperconducting junctions of the superconducting flux quantuminterference devices in said respective stages have almost equalcritical current values, a predetermined bias current is supplied to thesuperconducting junctions of the superconducting flux quantuminterference devices in said respective stages, and a flux quantumsignal is inputted to the superconducting flux quantum interferencedevices in the first stage to obtain an output voltage from thesuperconducting flux quantum interference devices in the third stage. 4.The superconducting driver circuit for voltage amplification accordingto claim 3, wherein the superconducting flux quantum interferencedevices are provided in four or more stages, and an output voltage isobtained from the superconducting flux quantum interference devices inthe uppermost stage.
 5. The superconducting driver circuit for voltageamplification according to claim 3, wherein the number of squids in theuppermost stage is one, and the number of superconducting flux quantuminterference devices constructing the respective stages is increased byone toward a lower stage.
 6. A superconducting driver circuit forvoltage amplification comprising: a superconducting flux quantuminterference device in a first stage constructing a closed loop havingas components two superconducting junctions and an inductor; asuperconducting flux quantum interference device in a second stageconstructing a closed loop having as components two superconductingjunctions and an inductor by sharing said inductor; and asuperconducting flux quantum interference device in a third stageconstructing a closed loop having as components two superconductingjunctions and an inductor by sharing the inductor of saidsuperconducting flux quantum interference device in said second stage,wherein the superconducting junctions of the superconducting fluxquantum interference devices toward a lower stage of the superconductingjunctions of the superconducting flux quantum interference devices insaid respective stages have a larger critical current value, apredetermined bias current is supplied to the superconducting junctionsof the superconducting flux quantum interference devices in saidrespective stages except for the superconducting junctions of thesuperconducting flux quantum interference device in the first stage, anda flux quantum signal is inputted to two superconducting junctions ofthe superconducting flux quantum interference device in the first stageto obtain an output voltage from the superconducting flux quantuminterference device in the third stage.
 7. The superconducting drivercircuit for voltage amplification according to claim 6, wherein thesuperconducting flux quantum interference devices are provided in fouror more stages, and an output voltage is obtained from a superconductingflux quantum interference device in the uppermost stage.
 8. Thesuperconducting driver circuit for voltage amplification according toclaim 6, wherein one input signal is provided to one superconductingjunction of the superconducting flux quantum interference device in thefirst stage, another input signal is provided to another superconductingjunction, the superconducting driver circuit is switched to the voltagestate by one input signal to hold a voltage state, and another inputsignal returns the superconducting driver circuit to a zero-voltagestate.
 9. A superconducting circuit comprising: a superconducting fluxquantum circuit performing predetermined signal processing using a fluxquantum as a signal carrier; a superconducting flux quantum-voltageconverter circuit which inputs a flux quantum signal outputted from thesuperconducting flux quantum circuit to output, for each flux quantumsignal input, a repeat state of a state that a flux quantum train existsand a state that a flux quantum train does not exist; and asuperconducting driver circuit for voltage amplification receiving as aninput the output of the superconducting flux quantum-voltage convertercircuit.
 10. The superconducting circuit according to claim 9, wherein acircuit which multiplies a flux quantum inputted as the output of saidsuperconducting flux quantum-voltage converter circuit to be dividedlyflowed to two circuits for being inputted to one output circuit isinserted between the superconducting flux quantum-voltage convertercircuit and the superconducting driver circuit for voltageamplification.